I enjoyed the episode as well and the fact that the 3rd girl ended up being the killer was a surprise. After the male suspect ended up not being the perpetrator (just guilty of the blackmail), I thought it was going to be Sarah Jane Morris's character, since it seemed like she was going to be one of the recognizable guest stars that always end up being the killer. In fact, the moment I saw her on the screen earlier in the episode, I said "She must be the killer!" .
I'm obviously out of touch. I almost never recognize the recognizable guest star.